The invention relates to accelerometers. More particularly, the invention relates to accelerometer calibrators.
A Super Shaker (FIG. 1) is a dual coil, vibration generator, used for accelerometer calibration, configured to operate in the horizontal direction. A description of the Super Shaker appears in B. F. Payne and G. B. Booth, “The NIST Super Shaker Project,” Proc. Metrologie 95, Nimes, France, pp. 296-301, (Oct. 16-19, 1995). The generator has a central longitudinal axis 500. The moving element 20 is held for axial movement relative to a fixed/stationary member by means of a first pair of flexures 22A and a second pair of flexures 22B. The moving element 20 includes a structural member 24 extending from a first (left in the illustration) rim 26A to a second rim 26B. Near the first rim 26A, the structural member 24 carries a first coil 28A. Near the second rim 26B, the structural member carries a second coil 28B. The fixed/stationary member includes first and second magnets 30A and 30B respectively cooperating with the coils 28A and 28B. Each exemplary magnet includes an inner pole 32 and an outer pole 34. Exemplary magnets are formed with an iron core and a permanent magnet stack. Other configurations, including electromagnets, are possible. Conical specimen mounting tables 34A and 34B are placed at the ends of deep central cavities 36A and 36B allows the mounting of accelerometers 40 and calibration devices close to one-another.
The moving element is driven by currents in the coils on the two ends provided by a low distortion audio amplifier, similar to amplifiers commonly used to drive loudspeakers used to reproduce music. In an exemplary coupling, the leads for one of the coils pass along the respective flexures of one flexure pair while the leads for the other pass along the flexures of the other flexure pair. Leads for the accelerometer may similarly pass to the associated accelerometer electronics (not shown). To generate the maximum vibratory force, the coils are connected in series, so that the total vibratory force is the sum of the vibratory forces generated at the two ends. They may also be used one-at-a-time. The primary use of the Super Shaker, for calibration, to date, has been the frequency range 5 Hz to 10,000 Hz, with occasional use to 15,000 Hz. Calibrations are commonly made in either of two generic ways, by Reciprocity, where the magnitude of the mass of a weight/mass 42 is changed, and the calibration calculated from the difference in acceleration for known differences in mass. This may involve a series of specific masses at each of a series of specific frequencies across the calibration range to provide a frequency-dependent calibration (correction) factor. Exemplary frequencies may start at the bottom of the range and increase from one to the next by a given percentage (e.g., 50%).
Alternatively, Interferometry is used, where the mass 42 is replaced by a mirror, on which an axial beam of known frequency is reflected, and the calibration calculated from the interference of the input beam and the reflected beam as measured by a photocell. Specifically, for each of the series of frequencies, the doppler effect is used to measure displacement/velocity/acceleration which is compared to the accelerometer output to provide a frequency-dependent calibration. A reference acceleration may be chosen based upon the accelerometer specification and calibrator performance. For example a 2 g reference acceleration may be sufficiently within the accelerometer envelope to generate a useful signal while not being so high as to cause problems generating the acceleration (e.g., calibrator heating that effects accuracy, etc.). The particular model of acceleration may have a known nominal characteristic relation between acceleration and accelerometer output. The calibration is required to correct for variations particular to the individual accelerometer specimen. An oscillator is used to provide a small voltage oscillating signal at the desired frequency. The amplifier is then adjusted to provide an output current at a level where the accelerometer output corresponds at least approximately to the reference acceleration in view of the nominal characteristic. The measured acceleration is compared to the output to generate the calibration.
Like reference numbers and designations in the various drawings indicate like elements.